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The Interdependence of Living Things

A queen bee © Paul Billiet

Animals and plants share the biosphere with each other. Many organisms live in groups and some of these groups are highly organized. Other organisms have very close associations with each other, some living in or on the bodies of others. All animals ultimately depend on the green plants, the producers, for their food. Humans share the biosphere with all the other organisms; the only difference is that humans have the ability to influence their environment.

Social Behaviour of Animals

Many species of animals are found living in large groups. We use special nouns, called collective nouns, to describe some of these groups. We talk about a herd of gnus, a shoal of herring, a school of dolphins and a flock of geese.

Temporary groups of animals

Flock of Cape Pigeons, Antarctica © Shirley Burchill

Some large groups of animals are only temporary groups. Frogs which gather together at a pool during the mating season and birds which migrate in large numbers are two examples of temporary groups.

One of the largest groups of animals, a swarm of locusts, is only a temporary group. It is impossible to tell when a swarm of locusts will appear in the sky; many years can go by without the locusts swarming at all.

It is thought that swarming occurs because of overcrowding in the mountain areas where locusts usually live. The young locusts, (called "hoppers" because they do not have wings), move off together. Eventually they mature into flying adults. The number of locusts in the swarm is gradually reduced as many of them are eaten by predators or are killed by bad weather conditions. After some time, those which are left will separate and start to live on their own.

Permanent groups of animals

Impala, Kenya © Shirley Burchill

There are many more permanent large animal groups. Herds of gnu and zebra wander across the African Savannah in search of food. These animals stay in large groups for their own protection. It is true that a carnivore is more likely to see a large group of herbivores feeding together than a single herbivore. From the point of view of the herd, however, it is much more likely that the carnivore will be seen by one of its members. This means that each herbivore is able to spend more time eating and less time looking around for danger.

Also, when a carnivore attacks a herd, there is less chance that any individual will be caught. This is a little difficult to understand, so let us look at it in another way.

Drawing illustrating the advantages of living in a group © Shirley Burchill A solitary gnu has little chance of escaping from a predator.
   
Drawing illustrating the advantages of living in a group © Shirley Burchill If a herd of gnus is attacked by a predator, each individual has less chance of being caught.

One gnu eating on its own has a 100% chance of being caught by a predator. If there are two gnus, however, they each have a 50% chance of being caught. In a herd of 100 gnus, each gnu has a 1% chance of being caught by the predator. The more gnus there are in the herd the less likely it is that one of them will be caught.

Also, the predator is often "confused" with so much choice. If the predator hesitates over its choice of prey, the prey may gain enough time to escape.

Of course, if there is food to eat animals will group together. The gnu and zebra are in herds because they are all on the savannah plains to eat the grass which grows there. Pigeons will flock together in a street if someone starts to feed them. Finally, it is much easier to find a mate if the animal is part of a group than if it lives on its own.

Adélie penguin colony, Antarctica © Shirley Burchill

These large groups of animals are not really showing social behaviour. In fact, their reasons for being in a group are completely selfish. They are together in the same place because it offers each one of them food, protection and an available mate when the mating season comes around. In order for a group of animals to be truly social it has to show co-operation between its members and a division of labour. (Division of labour means that the jobs to be done are given to different members of the group.)

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